Oracle has had this functionality for a loooong time. As a result, you might want to look at how Oracle databases use the DDL (and other) System level triggers.

For myself, I have created a DDL trigger in Oracle's SYS schema to record all changes including what object changed when, and by whom. As well, I have LOGIN/LOGOFF triggers that can be used to track who logged in, from where and if they changed any data (track COMMITs/rollbacks from Oracle's DMV)

John Kanagaraj